Spot-size transformer, method of producing spot-size transformer and waveguide-embedded optical circuit using spot-size transformer

ABSTRACT

The present invention is to provide a spot-size transformer which can transform the beam spot-size in a waveguide separated by a groove and a waveguide-embedded optical circuit using the spot-size transformer. The spot-size transformer according to the present invention comprises a first optical waveguide having a first core and a first cladding covering substantially the whole surface of the first core, a second optical waveguide having a second core provided as an extension of the first cladding and a second cladding, a transition waveguide positioned between the first and second optical waveguides, the transition waveguide having a first core whose width of the extension becomes gradually narrower as it goes toward the second optical waveguide.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.10/678,981, filed Oct. 3, 2003, now pending, which application isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

The present invention relates to a spot-size transformer and a method ofproducing the same, more particularly, to a low-loss spot-sizetransformer and a method of producing the same. Further, the presentinvention relates to a waveguide-embedded optical circuit using thespot-size transformer, and particularly to a waveguide-embedded opticalcircuit that has low loss and can be fabricated at low cost.

DESCRIPTION OF THE PRIOR ART

In recent years, optical communication is widely utilized to transmitinformation at high data rate. In optical communications, an opticalisolator element, an optical filter or the like is suitably inserted ata predetermined part of an optical waveguide (optical fiber or embeddedtype optical waveguide) to constitute an optical circuit.

FIG. 1 shows a ferrule for connecting optical fibers that is one kind ofoptical waveguide, and FIG. 1(a) is a schematic perspective of theferrule. FIG. 1(b) is a cross sectional view taken along line D-D ofFIG. 1(a). As shown in FIGS. 1(a) and (b), a ferrule 10 is used toconnect two optical fibers 11, 12, and a groove 10 a is formed in theregion of the junction between the two optical fibers. The optical fiber11 comprises an bare fiber 11 c consisting of a core 11 a and a cladding11 b covering the core 11 a, and a jacket 11 d covering the bare fiber11 c. Similarly, the optical fiber 12 comprises an bare fiber 12 cconsisting of a core 12 a and a cladding 12 b covering the core 11 a,and a jacket 12 d covering the bare fiber 12 c. Inside the ferrule 10,jackets 11 d, 12 d of the optical fibers 11, 12, are removed to exposethe bare fibers 11 c, 12 c. The bare fiber 11 c terminates at one sidewall portion of the groove 10 a and the element fiber 12 c terminates atthe other side wall portion of the groove 10 a. That is, the end face ofthe element fiber 11 c and the end face of the bare fiber 12 c areopposed to each other across the groove 10 a formed in the ferrule 10.

FIG. 2 shows an optical filter installed in the ferrule 10. FIG. 2(a) isa schematic perspective view thereof and FIG. 2(b) is a cross sectionalview taken along line E-E of FIG. 2(a). As shown in FIGS. 2(a) and (b),light entering from one end of the optical fibers 11, 12 is filtered bythe optical filter 30 inserted into the groove 10 a in accordance withthe filter characteristics and exits from the other end of the opticalfibers 11, 12. It is therefore possible to extract a light of desiredwavelength.

FIG.3 shows a waveguide-embedded optical circuit, constituting a kind ofan optical waveguide. FIG. 3(a) is a schematic perspective view thereofand FIG. 3(b) is a cross sectional view taken along line F-F of FIG.3(a). As shown in FIGS. 3(a) and (b), the waveguide-embedded opticalcircuit 20 comprises a substrate 21, a cladding layer 22 formed on thesubstrate 21 and a core region 23 formed in the cladding layer 22. Thecladding layer 22 and the core region 23 are separated by a groove 24into a part composed of a cladding layer 22 a and a core region 23 a,and a part composed of a cladding layer 22 b and a core region 23 b. Thecore region 23 a therefore terminates at one side wall portion of thegroove 24 and the core region 23 b terminates at the other side wallportion of the groove 24. That is, the end face of the core region 23 aand the end face of the core region 23 b are opposed to each otheracross the groove 24.

FIG. 4 shows an optical filter installed in the waveguide-embeddedoptical circuit 20. FIG. 4(a) is a schematic perspective thereof andFIG. 4(b) is a cross sectional view thereof taken along line G-G ofFIGS. 4(a). As shown in FIG. 4(a) and (b), light entering from one endof the core regions 23 a, 23 b is filtered by the optical filter 30inserted into the groove 24 formed in the cladding 22 and exits from theother end of the core region 23 a, 23 b. It is therefore possible toextract a light of desired wavelength.

Further, a non-reciprocal optical functional element, such as an opticalisolator element, is generally constituted to have an optical isolator,such as a faraday rotator or polarizer, positioned between two lenses.Similarly, an optical filter is also generally constituted to have anoptical filter element positioned between the two lenses. Suchconfigurations are well known in the art (see, for example, JP10-68910A,JP09-68660A). However, since a so-configured optical circuit has manyparts, it is difficult to miniaturize, and since it needs high precisionalignment of the optical axis, the production cost becomes high. Thus,the optical circuit of the waveguide-embedded type, in which the opticalfunctional element is directly inserted in a groove formed by separatingthe optical waveguide without lenses, has attracted attention.

However, when light propagates through the optical waveguide separatedby the groove, loss occurs that is caused mainly by diffraction in theseparated region.

FIG. 5 is a diagram for explaining this loss, and schematically showsthe state of light propagation from an optical waveguide 41 consistingof a core 41 a and a cladding 41 b across a gap to an optical waveguide42 consisting of a core 42 a and a cladding 42 b, wherein FIG. 5(a)shows the case of a small core size and FIG. 5(b) shows the case oflarge core size. As shown in FIGS. 5(a) and (b), since the light exitingthe optical waveguide spreads owing to diffraction, diffraction lossincreases as the gap “d” becomes larger. On the other hand, as can beseen from a comparison of FIG. 5(a) and FIG. 5(b), since the diffractionbecomes very pronounced as the beam-spot becomes smaller, it isnecessary to make the gap width narrow and enlarge the diameter of beamspot in order to reduce diffraction loss.

For this reason, when connecting two optical fibers using a ferrule,loss resulting from diffraction can be reduced if the spot-size istransformed by using a TEC (Thermally Expanded Core) fiber, i.e., afiber whose core diameter has been locally expanded at the end. As iswell known, the core in a TEC fiber is expanded by heating with amicro-burner, heater or the like. This is described in, for example,“Efficient coupling of a semiconductor laser to an optical fiber bymeans of a tapered waveguide on silicon” (Appl. Phys. Lett. 55(23), 4Dec. 1989, pp 2389-2391), “Polymeric buried core adiabatic opticalspot-size transformer” (ELECTRONICS LETTERS Vol. 38, No. 7, 28th Mar.2002, pp 319-321) and “Photoinduced refractive index change in B and Gecodoped SiO₂ formed by TEOS-PECVD method” (The Japan Society of AppliedPhysics Digest 2a-ZF-3, September 1999, p 1021).

However, since the heat capacity of the waveguide-embedded opticalcircuit shown in FIGS. 3 and 4 is very large compared with that of anoptical fiber, it is difficult to expand the diameter of a core locallyby heating in the manner of a TEC fiber. Thus, in this kind of opticalwaveguide, there is a problem that the loss owing to the diffractionthat arises in the groove in which the optical filter is insertedbecomes large.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide aspot-size transformer which can transform the beam spot-size in awaveguide separated by a groove and a waveguide-embedded optical circuitusing the spot-size transformer.

Another object of the present invention to provide a spot-sizetransformer that can transform the beam spot-size with low loss in awaveguide and a waveguide-embedded optical circuit using the spot-sizetransformer.

Further object of the present invention is to provide a method foreasily forming a spot-size transformer in a waveguide-embedded opticalcircuit.

A spot-size transformer according to the present invention comprises afirst optical waveguide having a first core and a first claddingcovering substantially the whole surface of the first core, a secondoptical waveguide having a second core provided as an extension of thefirst cladding and a second cladding, a transition waveguide positionedbetween the first and second optical waveguides, the transitionwaveguide having a first core whose width of the extension becomesgradually narrower as it goes toward the second optical waveguide.

According to the present invention, since the first cladding whichbelongs to the first optical waveguide is used as the second core whichbelongs to the second optical waveguide with the transition waveguide,it is possible to expand the spot-size of the beam which propagates fromthe first optical waveguide to the second optical waveguide.Furthermore, since the first cladding covers substantially the wholesurface of the first core, the center of the beam spot which propagatesthough the first optical waveguide and the center of the beam spot whichpropagates through the second optical waveguide does not displacedgreatly, so that it is possible to transform beam spot-size with lowloss.

In a preferred aspect of the present invention, each of the firstoptical waveguide and the second optical waveguide is channel-type.

In a further preferred aspect of the present invention, the center ofthe first core and the center of the second core are positionedsubstantially on the same axis. According to this aspect of the presentinvention, since the center of the beam spot which propagates in thefirst optical waveguide and the center of the beam spot which propagatesin the second optical waveguide are substantially coincide, it ispossible to transform the beam spot size more efficiently.

In a further preferred aspect of the present invention, the firstcladding has at least a lower cladding positioned under the first coreand an upper cladding positioned above the first core, and the bottom ofthe first core is in contact with the lower cladding and the uppersurface and the both sides of the first core are in contact with theupper cladding.

In a further preferred aspect of the present invention, the end face ofthe second core is substantially rectangular. And the section with thefirst cladding and the part constituting the second core that is anextension thereof substantially perpendicular to the axis arerectangular.

In a further preferred aspect of the present invention, the first corehas a shape obtained by omitting the end of the part that becomesgradually narrower. According to this aspect of the present invention,it is possible to reduce the fluctuation of in the characteristic causedby the fabrication conditions.

In a further preferred aspect of the present invention, the secondcladding is formed of ladder silicone or a silica glass. According tothis aspect of the present invention, it is possible to prevent thestrain induced by difference in thermal expansion coefficient betweenthe first cladding and the second cladding. And it is possible toprevent a change of the difference of the refractive indexes of thefirst cladding and the second cladding induced by temperaturedependency.

In a further preferred aspect of the present invention, the secondcladding is formed using a thin film process selected from a groupconsisting of a CVD process, a sputtering process, a vacuum depositionprocess, a FHD process and a sol-gel process.

A spot-size transformer according to the present invention comprises afirst optical waveguide having a first core and a first claddingcovering the first core, a second optical waveguide having a second coreand a second cladding covering the second core, a transition waveguidewhich is positioned between the first and the second optical waveguide,wherein a light propagated into the first waveguide has a first opticalfield distribution, a light propagated into the second waveguide has asecond optical field distribution, the transition waveguide changes fromthe first optical photoelectric field to the second optical fieldgradually or changes from the second optical field to the first opticalfield gradually; and the second core covers the first core at least inthe part corresponding to the transition waveguide and includes theregion where a refractive index is changing by irradiating energy beam.

According to the present invention, since the spot size of the beamwhich propagates from the first optical waveguide to the second opticalwaveguide can be expanded, it is possible to reduce a refractive losssignificantly by arranging an optical functional element on the side ofthe second optical waveguide. Moreover, since the second core includesthe region where the refractive index is changing by irradiating energybeam, it can be produced by a comparatively easy fabrication process.

In a preferred aspect of the present invention, the width of the part ofthe first core correspond to the transition waveguide becomes graduallynarrower as it goes toward the second optical waveguide. The part of atleast the first cladding is provided as the extension of the secondoptical waveguide. In a further preferred aspect of the presentinvention, the second cladding has a first part which consists ofsubstantially non-doped silica glass and a second part which consists ofsilica glass containing at least germanium (Ge). The second part furthercontains a first element which reduces refractive index, and therefractive indexes of the first part and the second part aresubstantially equal. In a further preferred aspect of the presentinvention, the first core consists of a material in which at leastgermanium (Ge), a first element and a second element which raise itsrefractive index are contained in the silica glass. The first element isboron (B) and the second element is phosphorus (P).

In a further preferred aspect of the present invention, the firstoptical waveguide and the second optical waveguide are channel-type andthe center of the first core and the second core are locatedapproximately on the same axis. Since the center of the beam spot hardlydisplaces in the above transition waveguide, it is possible to minimizethe loss arising in the transition waveguide.

A waveguide-embedded optical circuit according to the present inventioncomprises each of the first spot-size transformer and the second tspot-size transformer including at least a first optical waveguidehaving a first core and a first cladding, a second optical waveguidehaving a second core which is provided as the extension of the firstcladding and a second cladding, the second optical waveguide of thefirst spot-size transformer and the second optical waveguide of thesecond spot-size transformer, which face each other through a groove.

According to this aspect of the present invention, after the incidentlight in the first optical waveguide which is the first spot-sizetransformer propagates to the second waveguide whose beam spot isexpanded, the incident light propagates to the second waveguide which isthe second spot-size transformer, which face each other across a groove,and the beam spot is reduced again, and the incident light propagates inthe first optical waveguide. Since the beam spot of the lightpropagating through the groove is expanded, it is possible to reducerefractive loss significantly.

In a further preferred aspect of the present invention, thewaveguide-embedded optical circuit further comprises an opticalfunctional element which is inserted in the groove. According to thepresent invention, it is possible to extract a light of desiredwavelength with low-loss.

In a further preferred aspect of the present invention, each of thefirst spot-size transformer and the second spot-size transformercomprises a transition waveguide which is positioned between the firstand the second optical waveguide, the transition waveguide having thefirst core whose width of the part becomes gradually narrower as it goestoward the second optical waveguide. Further, the first cladding coverssubstantially the whole surface of the first core. The center of thefirst core and the center of the second core are positionedsubstantially on the same axis.

Further, a waveguide-embedded optical circuit according to the presentinvention comprises a pair of the spot-size transformers mentionedabove, the second optical waveguide of one spot-size transformer and thesecond optical waveguide of the other spot-size transformer, which faceeach other across a groove. According to this aspect of the presentinvention, it is possible to reduce refractive loss in the groovesignificantly. Therefore, if an optical filter or an optical isolatorelement is inserted in the groove, a low-loss waveguide-embedded opticalcircuit can be constituted. If several pairs of the waveguide-embeddedoptical circuit are utilized, an arrayed low-loss waveguide-embeddedoptical circuit can be constituted.

The present invention further provides a method of producing a spot-sizetransformer which comprises a first optical waveguide, a secondwaveguide and a transition waveguide positioned between the firstoptical waveguide and the second optical waveguide, which methodcomprises the steps of forming a region of the core corresponding to thepart of the first optical waveguide and the transition waveguide,forming a cladding layer covering at least the region of the secondoptical waveguide and the transition waveguide, and changing therefractive index gradually by projecting high-energy beam onto at leasta part of the cladding layer corresponding to the transition waveguideand the second optical waveguide.

In a preferred aspect of the present invention, the step of forming theregion of the core includes the steps of forming a core layer, andpatterning the core layer so that the width of the core layer issubstantially constant in the part corresponding to the first opticalwaveguide and becomes gradually thinner as it goes toward the secondoptical waveguide in the part corresponding to the transition waveguide.In a further preferred aspect of the present invention, the region inwhich the refractive index is change by irradiation with the high-energybeam comprises the core of the second optical waveguide and at least apart of the cladding of the first optical waveguide.

According to this aspect of the present invention, since the size of thebeam spot is changed without substantially displacing the center of thebeam spot in the channel type optical waveguide, it is possible totransform the spot-size at a low-loss.

Moreover, according to the present invention, since the first claddingis changed to the second core gradually by using a transition waveguide,the spot-size transformer can be produced by a comparatively easyfabrication process.

Furthermore, according to the present invention, since the region whichis the first cladding as well as the second cladding (refractive indexchanging region) is formed by irradiating ultraviolet rays, thespot-size transformer can be produced by a comparatively easyfabrication process.

Furthermore, according to the present invention, since two spot-sizetransformers face each other across a groove and the beam spot isexpanded in the groove, it is possible to reduce refractive losssignificantly.

Furthermore, according to the present invention, since the beam spot istransformed by the transition waveguide, it is possible to reducerefractive loss significantly in the groove by applying the presentinvention to the optical waveguide pair separated by the groove.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a ferrule which connects the optical fiber which is a kindof an optical waveguide and FIG. 1(a) is a schematic perspective andFIG.1(b) is a cross sectional view taken along line D-D of FIG. 1(a).

FIG. 2 shows the state where a optical filter is equipped in the ferrule10 and FIG. 2(a) is a schematic perspective and FIG. 2(b) is a crosssectional view taken along line E-E of FIG. 2(a).

FIG. 3 shows a waveguide-embedded optical circuit which is a kind of anoptical waveguide and FIG. 3(a) is a schematic perspective and FIG. 3(b)is a cross sectional view taken along line F-F of FIG. 3(a).

FIG. 4 shows the state where a optical filter is equipped in awaveguide-embedded optical circuit20 and FIG. 4(a) is a schematicperspective and FIG. 4(b) is a cross sectional view taken along line G-Gof FIG. 4(a).

FIG. 5 is a explanation about the loss which originates in diffractionphenomena occurs and FIG. 5(a) shows the case of small core size andFIG. 5(b) shows the case of large core size.

FIG. 6 is a schematic perspective view showing a waveguide-embeddedoptical circuit 100 that is a preferred embodiment of the presentinvention seen from one direction.

FIG. 7 is a schematic perspective view showing a waveguide-embeddedoptical circuit 100 that is a preferred embodiment of the presentinvention seen from opposite direction.

FIG. 8 is a cross sectional view taken along line A-A of FIG. 6.

FIG. 9 is a cross sectional view taken along line B-B of FIG. 6.

FIG. 10 is an expanded plain view which shows the end of the core region104-1 and 104-2.

FIG. 11 shows in detail configuration between the lower cladding layer102-2, 102-5 and the upper cladding layer 103-2, 103-5, and the coreregion 104-1, 104-2 in the waveguide-embedded optical circuit 100.

FIG. 12 is a schematic perspective view schematically showing the statewhere the optical filter 110 is equipped in the waveguide-embeddedoptical circuit 100

FIG. 13 is a cross sectional view taken along line C-C of FIG. 12.

FIG. 14 is a schematic perspective view which shows a state of thewaveguide-embedded optical circuit 100 in fabrication process.

FIG. 15 is a schematic perspective view which shows a state of thewaveguide-embedded optical circuit 100 in fabrication process.

FIG. 16 is a schematic perspective view which shows a state of thewaveguide-embedded optical circuit 100 in fabrication process.

FIG. 17 is a schematic perspective view which shows a state of thewaveguide-embedded optical circuit 100 in fabrication process.

FIG. 18 is a schematic perspective view which shows a state of thewaveguide-embedded optical circuit 100 in fabrication process.

FIG. 19 is a schematic perspective view which shows a state of thewaveguide-embedded optical circuit 100 in fabrication process.

FIG. 20 is a schematic perspective view showing the waveguide-embeddedoptical circuit 100A that is the other preferred embodiment of thepresent invention.

FIG. 21 is a schematic perspective view showing the waveguide-embeddedoptical circuit 100B that is the other preferred embodiment of thepresent invention.

FIG. 22 is a schematic perspective view showing the waveguide-embeddedoptical circuit 100C that is the other preferred embodiment of thepresent invention.

FIG. 23 is a schematic perspective view of a spot-size transformer 200according to a preferred embodiment of the present invention seen fromone side

FIG. 24 is a schematic perspective view of the spot-size transformer 200seen from opposite side.

FIG. 25 is a cross sectional view taken along line A-A of FIG. 23.

FIG. 26 is a cross sectional view taken along line B-B of FIG. 23.

FIG. 27 is an expanded plain view which shows the end of the core region204.

FIG. 28 shows a desirable configuration between the core region 204 andthe refractive index region 205 and a part of the end face 200 a shownin FIG. 23 on larger scale.

FIG. 29 shows a part of the fabrication process of the spot-sizetransformer 200.

FIG. 30 shows a part of the fabrication process of the spot-sizetransformer 200.

FIG. 31 shows a part of the fabrication process of the spot-sizetransformer 200.

FIG. 32 shows a part of the fabrication process of the spot-sizetransformer 200.

FIG. 33 shows a part of the fabrication process of the spot-sizetransformer 200.

FIG. 34 shows a part of the fabrication process of the spot-sizetransformer 200.

FIG. 35 shows a part of the fabrication process of the spot-sizetransformer 200.

FIG. 36 shows a part of the fabrication process of the spot-sizetransformer 200.

FIG. 37 is a schematic perspective view of the waveguide-embeddedoptical circuit 300.

FIG. 38 is a cross sectional view taken along line C-C of FIG. 37.

FIG. 39 is a cross sectional view taken along line D-D of FIG. 37.

FIG. 40 is a schematic perspective view schematically showing the statewhere the optical filter 302 is equipped in the waveguide-embeddedoptical circuit 300.

FIG. 41 is a cross sectional view taken along line E-E of FIG. 40.

FIG. 42 is a schematic perspective view which shows the external of theoptical isolator element 310.

FIG. 43 is a plain view which shows the arrayed waveguide-embeddedoptical circuit 400 which comprises several pair of the first embeddedoptical waveguide and the second embedded optical waveguide.

FIG. 44 is a plain view which shows the waveguide-embedded opticalwaveguide 500.

FIG. 45 is a graph which shows the optical field mode distribution ofthe beam inputted to the first optical waveguide in the example 1.

FIG. 46 is a graph which shows the optical field mode distribution ofthe beam outputted from the second optical waveguide in the example 1.

FIG. 47 is a graph which shows the optical field mode distribution ofthe beam inputted to the first optical waveguide in the example 3.

FIG. 48 is a graph which shows the optical field mode distribution ofthe beam outputted from the second optical waveguide in the example 3.

FIG. 49 is a graph which shows the relation between the totalirradiation energy of the KrF excimer laser and insertion loss.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Preferred embodiments of the present invention will now be explainedwith reference to the drawings.

FIG. 6 is a schematic perspective view showing a waveguide-embeddedoptical circuit 100 that is a preferred embodiment of the presentinvention seen from one direction, and FIG. 7 is a schematic perspectiveview showing the waveguide-embedded optical circuit 100 seen fromopposite direction.

As shown in FIG. 6 and FIG. 7, the waveguide-embedded optical circuit100 of this embodiment comprises a substrate 101, lower cladding layers102-1-102-6 (sometimes collectively called “lower cladding layer 102”),upper cladding layers 103-1-103-6 (sometimes collectively called “uppercladding layer 103”), core regions 104-1 and 104-2 (sometimescollectively called “core region 104”) and optical resin layers 105-1and 105-2 (sometimes collectively called “optical resin layer 105”) Thepart including the lower cladding layers 102-1-102-3, the upper claddinglayers 103-1-103-3, the core region 104-1 and the optical resin layer105-1 is separated by a groove 106 from the part including the lowercladding layers 102-4-102-6, the upper cladding layers 103-4-103-6, thecore region 104-2 and the optical resin layer 105-2.

The groove 106 is formed on the upper face of the substrate 101 toextend downward, making it possible to fully insert an optical filterdescribed later in detail so as to cover the whole cross-sectional areaof the core region 104, the upper cladding layer 103 and the lowercladding layer 102 with the filter.

The substrate 101 serves to ensure mechanical strength of thewaveguide-embedded optical circuit 100. The material of the substrate101 is not particularly limited insofar as the substrate 101 can ensurethe mechanical strength of the waveguide-embedded optical circuit 100and silicone or glass is preferably used as the material of thesubstrate 101.

The lower cladding layers 102-2, 102-5 and the upper cladding layers103-2, 103-5 serve as the “first cladding” and the “second core”described later in detail and their material is not particularly limitedinsofar as the refractive indexes of the substrate 101 and the coreregion 104 are different but silica glass or polymer is preferably usedas the material of the substrate 101 and the core region 104. Otherportions of the lower cladding 102 (the lower cladding layers 102-1,102-3, 102-4 and 102-6) and other portions of the upper cladding layer103 (the lower cladding layers 103-1, 103-3, 103-4 and 103-6) are formedof the same material as the lower cladding layers 102-2, 102-5 and theupper cladding layers 103-2, 103-5.

The core region 104 serves as “the first core” described later in detailand the material of the core region 104 is not particularly limitedinsofar as the refractive indexes of the lower cladding layer 102 andthe upper cladding layer 103 are different and silica glass or polymeris preferably used as the material of the core region 104. In the coreregion 104, the core region 104-1 is formed on part of the surface ofthe lower cladding layer 102 and the surface of the lower cladding layer102, and the core region 104-1 is covered with the upper cladding layer103-5.

The optical resin layer 105 serves as the “second cladding” describedlater in detail and the material of the optical resin layer 105 is notparticularly limited insofar as the refractive indexes of the lowercladding layer 102 and the upper cladding layer 103 are different and anultraviolet curable resin is preferably used as the material of the coreregion 104 in this embodiment.

FIG. 8 is a cross sectional view taken along line A-A in FIG. 6, andFIG. 9 is a cross sectional view taken along line B-B in FIG. 6. Asshown in FIG. 8 and FIG. 9, the core region 104-1, 104-2 hassubstantially fixed width (length of the up and down direction in FIG.8) over a given distance from the end face, and after that, the width ofthe end portion of the core region 104-1, 104-2 becomes graduallynarrower as it goes toward the groove 106. For this reason, the coreregion 104-1, 104-2 is not present between the lower cladding layer102-2, 102-5 and the upper cladding layer 103-2, 103-5 in the portionnear the groove 106, so that the upper cladding layer 103-2 is directlylaminated on the lower cladding layer 102-2 and the upper cladding layer103-5 is directly laminated on the lower cladding layer 102-5.

In the specification, the section where the width of the core regions104-1, 104-2 is set substantially uniform is called the “firstwaveguide”, the section where the core regions 104-1, 104-2 is notpresent is called the “second waveguide”, and the section where thewidth of the core regions 104-1, 104-2 becomes gradually narrower towardthe groove 106 is called the “transition waveguide”. The firstwaveguide, the transition waveguide and the second waveguide are unitedand are collectively called the “spot-size transformer”. Therefore, thewaveguide-embedded optical circuit 100 of this embodiment includes twospot-size transformers which face each other across the groove 106, andin each spot-size transformer, the second waveguide is positioned on theopposite side of the groove 106 (the opposite side is the end side ofthe waveguide-embedded optical circuit 100).

The first waveguide is a channel type optical waveguide constituted bythe first core and the first cladding, and the second optical waveguideis a channel type optical waveguide constituted by the second core andthe second cladding. As described above, each of the first cladding andthe second core is constituted by the layered members comprised of thelower cladding layer 102-2 and the upper cladding layer 103-2, or thelayered member comprised of the lower cladding layer 102-5 and the uppercladding layer 103-5. Moreover, the transition waveguides areconstituted by the region where the core changes from the first core tothe second core and the cladding changes from the first cladding to thesecond cladding. In this transition region, the spot-size of thepropagated beam changes from the relatively small spot-size in the firstoptical waveguide to a relatively large spot-size in the second opticalwaveguide (from relatively large spot-size in the second opticalwaveguide to relatively small spot-size in the first optical waveguide).That is, the actual spot size transformation is performed in thetransition waveguide.

It is preferable for the end of the taper portion of the core regions104-1, 104-2 to be sharper from the theoretical viewpoint of suppressingexcessive loss. However, from the viewpoint of actual fabrication, theshape with the end of the taper portion cut off as shown in FIG. 10 ispreferable. In this case, the width of the end part “x₁” is preferablyset to a small value within the range which does not vary greatlydepending on the fabrication conditions. Specifically, it is preferablyset to 1 μm or less, more preferably set to 0.6 μm or less. If the widthx₁ of the end part is set to 1 μm or less, it becomes possible tosuppress excessive loss to about 0.8 dB or less in most cases, and ifthe width x₁ of the end part is set to 0.6 μm or less, it becomespossible to suppress excessive loss to about 0.4 dB or less in mostcases. The length “x₂” of the tapered part is not limited but it ispreferably set between about 100 times and 200 times the width a₂ of theuntapered part (the region corresponding to the first optical waveguide)in the core region 104-1, 104-2. By these settings, it becomes possibleto prevent enlargement of the waveguide-embedded optical circuit 100 andeffectively suppress the excessive loss generated in the transitionwaveguide.

As shown in FIG. 8 and FIG. 9, the width (length of the up and down(vertical) direction in FIG. 8) and the height (length of the up anddown (vertical) direction in FIG. 9) of the layered member, which is thefirst cladding and the second cladding, comprised of the lower claddinglayer 102-2 and the upper cladding layer 103-2 has a substantially fixedwidth in the section from the end of face the waveguide-embedded opticalcircuit 100 to the groove 106. The same is true of the layered member ofthe lower cladding layer 102-5 and the lower cladding layer 103-5.

FIG. 11 shows in detail the configuration between the lower claddinglayers 102-2, 102-5 and upper cladding layers 103-2, 103-5, and the coreregion 104-1, 104-2 in the waveguide-embedded optical circuit 100.

As shown in FIG. 11, when the height of the core regions 104-1, 104-2 isset to a₁ and the height of the layered member comprised of the lowercladding layers 102-2 and the upper cladding layers 103-2, and thelayered member comprised of the lower cladding layer 102-5 and the uppercladding layer 103-5 is set to b₁, it is preferable to align the linewhich separates the core regions 104-1, 104-2 in the height directiona₁/2 and the line which separates the layered members in the heightdirection b₁/2 substantially coincide. That is, it is preferable toalign the center line in the height direction of the core regions 104-1,104-2 and the center line in the height direction of the layered memberssubstantially coincide. Similarly, when the width of the core region104-1, 104-2 is set to a₂ and the width of the layered members is set tob₂, it is preferable to align the line which separates the core region104-1, 104-2 in the height direction a₂/2 and the line which separatesthe layered members in the height direction b₂/2 substantially coincide.That is, it is preferable to align the center line in the widthdirection of the core regions 104-1, 104-2 and the center line in thewidth direction of the layered members substantially coincide. Thismeans that it is preferable to align the central point in the widthdirection of the core regions 104-1, 104-2 and the central point in thewidth direction of the layered members substantially coincide.

The height a₁, and width a₂ of the core regions 104-1, 104-2 are notlimited but are preferably set to almost the same size as the diameterof the core of an ordinary optical fiber (about 7 μm). When they are soset, it becomes possible to connect the first optical waveguide andoptical fiber directly by a groove of V shape etc.

In the waveguide-embedded optical circuit 100 having the above-describedconfiguration, after the light entering the first optical waveguidepropagates through the first core to the groove 106, it graduallypenetrate into the first cladding in the transition waveguide where thewidth of the first core becomes gradually narrower. Thus, in thetransition waveguide, the first cladding begins to serve as a secondcore as it goes toward the groove 106 and comes to serve almost totallyas a second core in the second optical waveguide. Therefore, the beamspot exiting from the groove 106 is expanded to larger than the beamspot entering the first optical waveguide. Further, after the lightentering the second optical waveguide propagates through the second coreto the opposite side from the groove 106, the incident light graduallypenetrate to the first cladding in the transition waveguide where thewidth of the first core becomes gradually wider. Thus, in the transitionwaveguide, the second core begins to serve as the first cladding as itgoes toward the opposite side from the groove 106 and comes to servealmost totally as the first cladding in the first optical waveguide.Therefore, the beam spot exiting from the first optical waveguide isreduced to smaller than the beam spot entering from the groove 106.

In the waveguide-embedded optical circuit 100 having the above-describedconfiguration, an optical filter can be inserted in the groove 106.

FIG. 12 is a schematic perspective view schematically showing the statewhere an optical filter 110 is installed in the waveguide-embeddedoptical circuit 100 and FIG. 13 is a cross sectional view taken alongline C-C of FIG. 12. As shown in FIG. 12 and FIG. 13, the beampropagated from one side of the core region 104-1, 104-2 is filtered inaccordance with the characteristic of the optical filter 110 insertedinto the groove 106 and propagates to the other side of the core regions104-1, 104-2. Thereby, it is possible to extract a light of desiredwavelength.

In the waveguide-embedded optical circuit 100 of this embodiment, sincethe portion separated by the groove 106 serves as a second opticalwaveguide with a core of larger diameter than the first opticalwaveguide, the beam spot propagating through the groove 106 is expandedto larger than the beam spot which propagates through the firstwaveguide. As a result, it is possible to significantly reduce thediffraction loss arising in the groove 106. Therefore, it becomespossible to achieve desired filtering at lower loss than with theconventional waveguide-embedded optical circuit shown in FIG. 3 and FIG.4. Further, in the waveguide-embedded optical circuit 100 of thisembodiment, as explained with reference to FIG. 11, it is possible tominimize the loss arising in the transition waveguide because the centerof the beam spot hardly displaces in the transition waveguide if thecenter section of the core regions 104-1, 104-2, which are the center ofthe first core, and the center section of the layered members, which isthe second core, are made coincident.

Next, the fabrication process of the waveguide-embedded optical circuit100 of this embodiment will be explained with reference to the drawing.

First, a substrate 101 of given area is prepared (FIG. 14) and the lowercladding layer 102 and the core region 104 are formed in order over thewhole surface of the substrate 101 (FIG. 15). The method of forming thelower cladding layer 102 and the core region 104 is not particularlylimited but a vapor phase growth process using chemical speciescontaining elements for forming the lower cladding layer 102 and thecore region 104 such as a CVD process, a sputtering process, a vacuumdeposition process, an FHD (Flame Hydrolysis Deposition) process, acoating process or the like is preferably used. Further, in the casewhere silica glass is used as the material of the lower cladding layer102 and the core region 104, the CVD process or the FHD process is morepreferably used from the viewpoint of productivity and quality. Further,in the case where polymer is used as the material of the lower claddinglayer 102 and the core region 104, a coating process is more preferablyused from the viewpoint of easy processing.

Next, the core regions 104-1, 104-2 are formed by patterning the coreregion 104 (FIG. 16). The core regions 104-1, 104-2 are formed to theshape mentioned above so as to provide the part where the width is fixedand the part where the width becomes gradually narrower. The method ofpatterning the core region 104 is not particularly limited but it ispreferable to form a metal mask layer over the whole surface of the coreregion 104, apply photoresist on the metal mask layer, form an etchingmask for leaving the core regions 104-1, 104-2, and remove theunnecessary portions of the core region 104 using the etching mask. Theremoval of the unnecessary portions of the core region 104 is preferablyperformed by dry etching.

Next, the upper cladding layer 103 is formed over the whole surface ofthe lower cladding layer 102 (FIG. 17). The method of forming the uppercladding layer 103 is not particularly limited, but the vapor phasegrowth process using chemical species containing elements for formingthe upper cladding layer 103 or coating is preferably used in the sameway as when forming the lower cladding layer 102 and the core region104. Further, as mentioned above, in the case where silica glass is usedas the material of the upper cladding layer 103, the CVD process or theFHD process is more preferably used. Further, as mentioned above, in thecase where polymer is used as a material of the upper cladding layer103, the coating process is more preferably used.

Next, three parallel rod-shaped members are formed by patterning thelayered members of the lower cladding layer 102 and the upper claddinglayer 103 (the core region 104 is partially included) (FIG. 18). Sincethe center rod-shaped member is used as the first optical waveguide andthe second optical wave guide (second core), it is necessary to controlthe size of rod-shaped member accurately. On the other hand, since thetwo rod-shaped members on the both sides are used as an outer frame ofthe optical resin layer 105, which is filled in the following processes,it is not necessary to control the size of the two rod-shaped members soaccurately as the center rod-shaped member. The method of patterning thelayered members of the lower cladding layer 102 and the upper claddinglayer 103 is not particularly limited, but it is preferable to form ametal mask layer over the whole surface of the upper cladding layer 103,apply photoresist on the metal mask layer, form an etching mask as toleave the three parallel rod-shaped members, and move the unnecessaryportions of the lower cladding layer 102 and the upper cladding layer103 using the etching mask. The removal of the unnecessary portions ofthe lower cladding layer 102 and the upper cladding layer 103 ispreferably performed by dry etching.

Further, the regions between the central rod-shaped member and therod-shaped members on the both sides are filled with the optical resinlayer 105 so as to cover the central rod-shaped member (FIG. 19), theoptical resin layer 105 is hardened, and the groove 106 is formed (FIG.6, FIG. 7). The method of forming the groove 106 is not particularlylimited, but it is preferably formed by dicing using a dicing machine.

In the waveguide-embedded optical circuit 100, after the light enteringthe first optical waveguide propagates through the first core toward thegroove 106, propagates through the transition waveguide and the secondoptical waveguide, exit at the groove 106, and then enters the secondoptical waveguide. Since the energy of the light entering the secondoptical waveguide is confined at a region 10 μm from the surface of thesecond core (first cladding), it is preferable for the thickness of theoptical resin layers 107-1, 107-2, 108-1, 108-2 serving as the secondcladding to be 10 μm or greater.

Other preferred embodiments of the present invention will now beexplained.

FIG. 20 is a schematic perspective view showing a waveguide-embeddedoptical circuit 100A that is another preferred embodiment of the presentinvention.

As shown in FIG. 20, the waveguide-embedded optical circuit 100A isdifferent from the waveguide-embedded optical circuit 100 shown in FIG.6 in the point that a ladder silicone is used for the optical resinlayers 107-1, 107-2. The waveguide-embedded optical circuit 100A is thesame as the waveguide-embedded optical circuit 100 shown in FIG. 6 inother aspects.

In the case of the functional group having siloxane as its main chainstructure generally contained in silicone, the optical characteristic ofthe waveguide are degraded owing to light absorption that occurs as afunction of the vibration mode and frequency. However, in thisembodiment, the functional group having siloxane as its main chainstructure contained in ladder silicone is removed by condensationoccurring when ladder silicone is heated. Therefore, degradation of theoptical characteristic of the waveguide caused by absorption of lightcan be prevented.

Moreover, since ladder silicone and silica glass have siloxane as theirmain chain structure, the thermal expansion coefficients of laddersilicone and silica glass are almost the same and the temperaturedependency of the refractive indexes of ladder silicone and silica glassare also almost the same. Therefore, when the lower cladding layer 102and the upper cladding layer 103 are formed of silica glass, the thermalexpansion coefficient of the center rod-shaped member and the opticalresin layers 107-1, 107-2 are almost the same and the temperaturedependency of the refractive indexes of the central rod-shaped memberand the optical resin layers 107-1, 107-2 is also almost the same.Consequently, the strain induced by difference in thermal expansioncoefficient at the boundary of the central rod-shaped member and theoptical resin layers 107-1, 107-2 can be prevented. And a change of thedifference of the refractive indexes induced by temperature dependencyat boundary of the center rod-shaped member and the optical resin layers107-1, 107-2 can be prevented.

As a shown FIG. 20, the waveguide-embedded optical circuit 100A isfabricated by coating and heating the paste of the ladder silicone onthe surface of the central rod-shaped member among the three parallelrod-shaped members shown in FIG. 18 and hardening the ladder silicone.

Another preferred embodiment of the present invention will now beexplained.

FIG. 21 is a schematic perspective view showing a waveguide-embeddedoptical circuit 100B that is another preferred embodiment of the presentinvention. The center rod-shaped member and the optical resin layers108-1, 108-2 are formed using the same silica glass. Consequently,strain induced by difference in thermal expansion coefficient at theboundary of the central rod-shaped member and the optical resin layers108-1, 108-2 can be prevented. And a change of the difference of therefractive indexes induced by temperature dependency at the boundary ofthe center rod-shaped member and the optical resin layers 108-1, 108-2can be prevented.

As a shown FIG. 21, the waveguide-embedded optical circuit 100B isdifferent from the waveguide-embedded optical circuit 100 shown in FIG.6 in the point that silica glass is used for the optical resin layer105. The waveguide-embedded optical circuit 100B is the same as thewaveguide-embedded optical circuit 100 shown in FIG. 6 in other aspects.

The lower cladding layer 102, the upper cladding layer 103, and theoptical resin layers 108-1, 108-2 are formed using the same silicaglass. Consequently, the strain induced by difference in thermalexpansion coefficient at the boundary of the central rod-shaped memberand the optical resin layers 108-1, 108-2 can be prevented. And thestrain induced by difference in the temperature dependency of therefractive indexes at the boundary of the center rod-shaped member andthe optical resin layers 108-1, 108-2 can be prevented.

As a shown FIG. 21, the waveguide-embedded optical circuit 100B isfabricated by forming a silica glass film on the surface of the centralrod-shaped member among the three parallel rod-shaped members shown inFIG. 18.

The silica glass film is formed using the CVD process, the sputteringprocess, the vacuum deposition process, the FHD process or the sol-gelprocess.

In the case where the silica glass film is formed by the CVD process, itis desirable to raise the temperature of the surface of the centralrod-shaped member to promote formation of the silica glass film. Thisenables reliable formation of the silica glass film over the wholesurface of the central rod-shaped member.

In the case where the silica glass film is formed by the sputteringprocess or the vacuum deposition process, it is desirable to use anapparatus having rotary and revolutionary mechanisms. This enables thesilica glass film to be uniformly formed over the whole surface of thecentral rod-shaped member by setting the substrate 101 formed with thecentral rod-shaped member in the apparatus equipped with the rotary andrevolutionary mechanisms, and rotating and revolving the substrate 101to deposit the silica glass vapor deposition particles on the surface ofthe central rod-shaped member.

In the case where the silica glass film is formed by the sol-gelprocess, it is desirable to form an amorphous silica film on the surfaceof the central rod-shaped member beforehand using liquid phasedeposition or the like. By forming the amorphous silica film, it ispossible to prevent cracking of the silica glass film that might becaused by volume contraction when the silica glass film is formed by thesol gel process.

Another preferred embodiment of the present invention will now beexplained.

FIG. 22 is a schematic perspective view showing a waveguide-embeddedoptical circuit 100C that is another preferred embodiment of the presentinvention.

As shown in FIG. 22, the waveguide-embedded optical circuit 100C of thisembodiment comprises the substrate 101, the lower cladding layers 102-2,102-6 (not shown), the upper cladding layers 103-2, 103-5 (not shown),the core regions 104-1-104-2 (not shown) and the optical resin layers109-1-109-4. The wave-guide embedded optical circuit 100c is separatedby a groove 106-1 into a part composed of the lower cladding layer102-2, the upper cladding layer 103-2, the core region 104-1 and theoptical resin layer 109-1, 109-2, and a part composed of the lowercladding layer 102-5, the upper cladding layer 103-5, the core region104-2 and the optical resin layers 109-3, 109-4.

The waveguide-embedded optical circuit 100C is fabricated as following.First, a substrate 101 of given area is prepared and the lower claddinglayer 102 and the core region 104 are formed in order over the wholesurface of the substrate 101. The core regions 104-1, 104-2 are formedby patterning the core region 104 and the upper cladding layer 103 isformed on the surface of the lower cladding layer 102 and the coreregion 104. One rod-shaped member is formed by patterning the layeredmembers of the lower cladding layer 102 and the upper cladding layer103. And the optical resin layers 109-1, 109-2 are formed to cover therod-shaped member.

The method of forming the optical resin layers 109-1, 109-3 is notparticularly limited, but it is desirable to form them by the CVDprocess using silica glass. The method of forming the optical resinlayers 109-2, 109-4 is not particularly limited, but it is desirable toform them using ladder silicone. A good optical resin layer can beobtained by forming the optical resin layers 109-1 and 109-3 of silicaglass. Moreover, the optical resin layers 109-1-109-4 can be formed inshorter time when the optical resin layer 109-2, 109-4 are formed ofladder silicone than when they are formed of silica glass.

The groove 106 is formed on the top surface of the substrate 101 toextend downward.

This completes the fabrication of the waveguide-embedded optical circuit100C of this embodiment. However, the method of producing thewaveguide-embedded optical circuit 100C of this embodiment is notlimited to the foregoing, and it can also be fabricated by othermethods.

A further preferred embodiment of the present invention will now beexplained with reference to the drawings.

FIG. 23 is a schematic perspective view of a spot-size transformer 200according to a preferred embodiment of the present invention seen fromone side, and FIG. 24 is a schematic perspective view of the spot-sizetransformer 200 seen from the opposite side. Though described later indetail, the spot-size transformer 200 of this embodiment is preferablyused as a component of s waveguide-embedded optical circuit.

As shown in FIG. 23 and FIG. 24, the spot-size transformer 200 comprisesa substrate 201, lower cladding layer 202, upper cladding layer 203, acore region 204, a refractive index changing region 205 and a topmostcladding layer 206, and the core region 204 and the refractive indexchanging region 205 which surrounds the core region 204 are exposed onone end face 200 a of the spot-size transformer 200 (see FIG. 23).

The substrate 201 ensures the mechanical strength of the spot-sizetransformer 200 and serves as a part of “the second cladding” describedlater in detail. In this embodiment, a non-doped silica glass (SiO₂) isused as the material of the substrate 201. The refractive index n of thesilica glass is 1.446.

The lower cladding layer 202 and the upper cladding layer 203 serve aspart of “the second cladding” described later in detail. In thisembodiment, germanium (Ge) and Boron (B)-doped silica glass (GBSG) isused as the material of the lower cladding layer 202 and the uppercladding layer 203. If germanium (Ge) is doped to silica glass, therefractive index increases, and if boron (B) is doped to silica glass,the refractive index decreases. Therefore, the refractive indexes of thelower cladding layer 202 and the upper cladding layer 203 become almostthe same as the refractive index (n=1.4460) of the substrate 201 whichconsists of non-doped silica glass. The germanium (Ge) is doped so thatpart of the lower cladding layer 202 and the upper cladding layer 203can be changed into a refractive index changing region 205 byirradiation of ultraviolet rays, and the boron (B) is doped in order toreduce the refractive index raised by doping of germanium (Ge) and matchthe index to the almost the same refractive index as the substrate 201.Therefore, the element doped together with the Germanium (Ge) can be anyelement that reduces the refractive index of silica glass, and, forexample, iron (Fe) may be used with or instead of boron (B).

The refractive index changing region 205 serves as “the first cladding”and “the second core” described later in detail, and it is formed byirradiating part of the lower cladding layer 202 and the upper claddinglayer 203 with ultraviolet rays. Although the refractive index changingregion 205 has the same composition as the lower cladding layer 202 andthe upper cladding layer 203, it has a refractive index (n=1.4485)higher than that of the lower cladding layer 202 and the upper claddinglayer 203 because refractive index increases when silica glass includinggermanium (Ge) is exposed to ultraviolet rays.

The core region 204 serves as “the first core” described later indetail, and germanium (Ge), boron (B) and phosphorus (P)-doped silicaglass (GBPSG) is used as the material of the core region 204 in thisembodiment. Since the core region 204 is covered with the refractiveindex changing region 205 and it includes germanium (Ge), the refractiveindex of the core region 204 after irradiation with ultraviolet raysincreases relative to that at the time of film forming. Further, sincethe refractive index of the silica glass increases when phosphorus (P)is doped, the refractive index of the core region 204 is higher than therefractive index changing region 205 which surrounds it (n=1.4517).However, in order to match the refractive indexes before and afterirradiation of ultraviolet rays in the core region 204 and therefractive index of the changing region 205 into agreement, it ispreferable to make the concentration of the germanium (Ge) doped to therefractive index changing region 205 and the concentration of thegermanium (Ge) doped to the core region 204 almost equal. In addition,it is more preferable to make the concentration of the boron (B) dopedin the refractive index changing region 205 and the concentration of theboron (B) doped in the core region 204 almost equal.

The topmost cladding layer 206 serves as a part of the second claddingdescribed later in detail, and non-doped silica grass (SiO₂) is used sthe material for the topmost cladding layer 206 in the embodiment. Asdescribed above, the refractive index of the non-doped silica glass is1.4460.

FIG. 25 is a cross sectional view taken along line A-A in FIG. 23, andFIG. 26 is a cross sectional view taken along line B-B of FIG. 23.

As shown in FIG. 25 and FIG. 26, the core region 204 has substantially afixed width (the vertical length of the up and down in FIG. 25) in thesection from the end face 200 a, and after that, the width of the endportion of the core region 204 becomes gradually narrower as it goestoward the end face 200 b. For this reason, the core region 204 is notpresent between the lower cladding layer 202 and the upper claddinglayer 203 in the portion near the end face 200 b and the two layers arecontacted each other there in a state of being laminated directly.

Moreover, the refractive index changing region 205 is formed to have afixed width (the vertical length of the up and down in FIG. 25) in thesection from the end face 200 a to the end face 200 b. However, it isnot necessary to form the refractive index changing region 205 inportion in which the width of the core region 204 is fixed, and it maybe formed in the section from the end face 200 b to the position 204 awhere the core region 204 is formed to taper. Moreover, the height (thevertical length of the up and down in FIG. 26) of the refractive indexchanging region 205 is as same as the height of the layered members ofthe lower cladding layer 202 and the upper cladding layer 203. Asdescribed above, the width and height of the refractive index changingregion 205 (the vertical length of the up and down in FIG. 26) is setsubstantially uniform.

In this specification, the section in the spot-size transformer 200where the width of the core region 204 is set substantially uniform iscalled “the first waveguide”, the section where the core region 204 isnot present is called “the second waveguide”, and the section where thewidth of the core region 204 becomes gradually narrower as it goestoward the end face 200 b is called “the transition waveguide”. That is,the spot-size transformer 200 of this embodiment comprises the firstoptical waveguide, the second waveguide and the transition waveguideprovided between the first optical waveguide and the second opticalwaveguide.

The first waveguide is a channel type optical waveguide constituted bythe first the core and first cladding, and the second optical waveguideis a channel type optical waveguide, constituted by the second core andthe second cladding. As mentioned above, each of the first cladding andthe second core is constituted by the refractive index changing region205. Moreover, the transition waveguide is the region where the corechanges from the first core to the second core and the cladding changesfrom the first cladding to the second cladding. In this transitionregion, the beam spot-size propagated changes from the relatively smallspot-size in the first optical waveguide to a relatively large spot-sizein the second optical waveguide (from relatively large spot-size in thesecond optical waveguide to relatively small spot-size in the firstoptical waveguide). That is, the actual the spot size transformation isperformed in the transition waveguide.

It is preferable for the end of the taper portion of the core region 204to be sharper from the theoretical viewpoint of suppressing excessiveloss. However, from the viewpoint of actual fabrication, a shape withthe end of the taper portion cut off as shown in FIG. 27 is preferable.In this case, the width of the end part “x₁” is preferably set to asmall value within the range which does not vary greatly depending onthe fabrication conditions. Specifically, it is preferably set to 1 μmor less, more preferably set to 0.6 μm or less. If the width x₁ of theend part is set to 1 μm or less, it becomes possible to suppressexcessive loss to about 0.8 dB or less in most cases, and if the widthx₁ of the end part is set to 0.6 μm or less, it becomes possible tosuppress excessive loss to about 0.4 dB or less in most cases. Thelength “x2” of the tapered part is not limited but it is preferably setbetween about from 100 times to 200 times the width a₂ of the untaperedpart (the region corresponding to the first optical waveguide) in thecore region 204. By these settings, it becomes possible to preventenlargement of the spot-size transformer and effectively suppress theexcessive loss generated in the transition waveguide.

FIG. 28 shows a desirable configuration between the core region 204 andthe refractive index region 205 and a part of the end face 200 a shownin FIG. 23 on larger scale.

As shown in FIG. 28, when the height of the core region 204 is set to a₁and the height of the refractive index changing region 205 is set to b₁,it is preferable to align the line which separates the core region 204in the height direction a₁/2 and the line which separates the refractivechanging region 205 in the height direction b₁/2 substantially coincide.That is, it is preferable to align the center line in the heightdirection of the core region 204 and the center line in the heightdirection of the refractive index changing region 205 substantiallycoincide. Similarly, when the width of the core region 204 is set to a₂and the width of the refractive index changing region 205 is set to b₂,it is preferable to align the line which separates the core region 204in the height direction a₂/2 and the line which separates the refractivechanging region 205 in the height direction b₂/2 substantially coincide.That is, it is preferable to align the center line in the widthdirection of the core region 204 and the center line in the widthdirection of the refractive index changing region 205 substantiallycoincide.

The height a₁ and width a₂ of the core region 204 is not limited but ispreferably set to almost the same size as the diameter of the core ofand ordinary optical fiber (about 7 μm). When they are so set, itbecomes possible to connect the first optical waveguide and opticalfiber directly by a groove of V shape etc.

In the waveguide-embedded optical circuit 200 having the above-describedconfiguration, after the light entering the end face 200 a of the firstoptical waveguide propagates through the first core to the end face 200b, gradually penetrates into the first cladding in the transitionwaveguide where the width of the first core becomes gradually narrower.Thus, in the transition waveguide, the first cladding begins to serve asthe second core as it goes toward the end face 200 b and comes to servealmost totally as the second core in the second optical waveguide.Therefore, the beam spot exiting from the end face 200 b is expanded tolarger than the beam spot entering the end face 200 a. Further, afterthe light entering the end face 200 b in the second optical waveguidepropagates through the second core to the end face 200 a, it graduallypenetrates to the first cladding in the transition waveguide where thewidth of the first core becomes gradually wider. Thus, in the transitionwaveguide, the second core begins to serve as the first cladding as itgoes toward the end face 200 a and comes to server almost totally as thefirst cladding in the first optical waveguide. Therefore, the beam spotexiting from the end face 200 a is reduced to smaller than the beam spotentering from the end face 200 b.

Next, the fabrication process of the spot-size transformer 200 of thisembodiment will be explained with reference to the drawing. However,since the spot-size transformer 200 of this embodiment is preferablyused as a component of a waveguide-embedded optical circuit, thespot-size transformer 200 is not necessarily separately fabricated.

First, the substrate 201 consisting of non-doped silica glass isprepared (FIG. 29) and the lower cladding layer 202 and core layer 204′are formed in order over the whole surface of the substrate 201 (FIG.30). As mentioned above, the lower cladding layer 202 consists ofgermanium (Ge) and boron (B)-doped silica glass (GBSG). The core layer204′ is a layer which becomes the core region 204 by patterning andconsists of germanium (Ge), boron (B), and phosphorus (P)-doped silicaglass (GPSG)). The method of forming the lower cladding layer 202 andthe core layer 204′ is not particularly limited but a vapor phase growthprocess using chemical species containing elements for forming the lowercladding layer 202 and the core layer 204′ such as a CVD process, asputtering process, a vacuum deposition process, an FHD or the like ispreferably used. Further, from the viewpoint of productivity andquality, the CVD method or the FHD method is more preferably used.

Next, the core region 204 is formed by patterning the core layer 204′(FIG. 31). The core region 204 is formed to the shape mentioned above soas to provide the part where the width is fixed and the part where thewidth becomes gradually narrower. The method of patterning the corelayer 204′ is not particularly limited but it is preferable to form ametal mask layer over the whole surface of the core layer 204′, applyphotoresist on the metal mask layer, form an etching mask for leavingthe core region 204, and remove the unnecessary portions of the corelayer 204′ using the etching mask. The removal of the unnecessaryportions of the core layer 204′ is preferably performed by dry etching.

Next, the upper cladding layer 203 is formed over the whole surface(FIG. 32). The method of forming the upper cladding layer 203 is notparticularly limited but a vapor phase growth process using chemicalspecies containing elements for forming the upper cladding layer 203such as a CVD process or an FHD process is preferably used by the samemethod as in forming the lower cladding layer 202 and the core claddinglayer 204′. Although unevenness corresponding to the core region 204appears on the surface of the upper cladding layer in the stateimmediately after film formation of the upper cladding layer 203, thesurface can be flattened by flowing during the annealing process. Then,the topmost cladding layer 206 is formed on the surface of the uppercladding layer 203 (FIG. 33). The method of forming the topmost claddinglayer 206 is not particularly limited but vapor deposition using achemical species containing the elements constituting the upper claddinglayer 203 is preferably used by the same method as in forming the lowercladding layer 202 and the like.

Next, the metal mask layer 207′ is formed on the surface of the topmostcladding layer 206 (FIG. 34) and the metal mask 207 is formed byremoving the metal mask layer 207′ where the transition changing region205 should be formed (FIG. 35). The material and thickness of the metalmask layer 207′ are not particularly limited insofar as metal mask layer207′ can substantially block ultraviolet rays. For example, WSi of athickness of about 1 μm can be used. The method forming the thin film ofthe metal mask layer 207′ is not particularly limited but vapordeposition using a chemical species containing the elements constitutingthe metal mask layer 207′ is preferably used. From the viewpoint ofproductivity, sputtering is preferably used. The method of patterningthe metal mask layer 207′ is not particularly limited but it ispreferable to form a metal mask layer over the whole surface of themetal mask layer 207′, apply photoresist on the metal mask layer, forman etching mask for leaving the metal mask 207, and remove unnecessaryportions of the metal mask layer 207′ using the etching mask. Theremoval of the unnecessary portions of the metal mask layer 207′ ispreferably removed by dry etching.

Next, a portion of the surfaces of the lower cladding layer 202 andupper cladding layer 203 that is not covered by the metal mask 207 isconverted into the refractive index changing region 205 by being exposedin ultraviolet rays through the patterned metal mask layer 207 (FIG.36). More specifically, since the germanium (Ge) was doped to the lowercladding layer 202 and upper cladding layer 203, the refractive indexincreases upon irradiation with ultraviolet rays. As a result, a part ofthe lower cladding layer 202 and the upper cladding layer 203 can bemade into the refractive index changing region 205 which has a highrefractive index. At this time, the ultraviolet rays are irradiated ontothe core region 204, so that the refractive index of the core region 204also increases together with that of the lower cladding layer 202 andthe upper cladding layer 203.

Then the metal mask 207 is removed to complete the spot-size transformer200 of this embodiment (FIG. 23).

As described above, the spot-size transformer 200 comprises the firstoptical waveguide, the transition waveguide and the second opticalwaveguide. It can expand the relatively small spot size of a beampropagating through the first optical waveguide send it to the secondoptical waveguide and reduce the relatively large spot size of a beampropagating through the second optical waveguide and send to the firstoptical waveguide. Further, since the portion that serves as the firstcladding and the second core (the refractive index changing region 205)is formed by irradiation of ultraviolet rays, the spot-size transformer200 can be fabricated with a comparatively easy process. Further, asexplained with reference to FIG. 28, it is possible to suppress the lossoccurring in the transition waveguide minimum because the center of thebeam spot hardly displaces in the transition waveguide if the center ofthe first core and the center of the second core (first cladding) arealigned.

Next, a waveguide-embedded optical circuit 300 using a pair of thespot-size transformers 200 will be explained.

FIG. 37 is a schematic perspective view of the waveguide-embeddedoptical circuit using a pair of the spot-size transformers 200-1, 200-2(optical filter circuit), and FIG. 38 is a cross sectional view takenalong line C-C of FIG. 37, and FIG. 39 is a cross sectional view takenalong line D-D of FIG. 37.

As shown from FIG. 37 to FIG. 39, the waveguide-embedded optical circuit300 has the structure wherein a pair of the spot-size transformer 200-1,200-2 are arranged so that the end faces 200 b face each other across agroove 301. The spot-size transformer 200-1, 200-2 has same structure asthe spot-size transformer 200 shown in FIG. 23 to FIG. 28 and can befabricated by same method as explained with reference to FIG. 29 to FIG.36. An optical functional element such as optical filter can be insertedinto the groove 301 of the waveguide-embedded optical circuit 300 ofthis configuration.

A groove 301 is so formed on the upper face of the substrate 201 toextend downward, that making it possible to insert an optical filter soas to covers the whole cross-sectional area of the core region 204, theupper cladding layer 203 and the lower cladding layer 202 with anoptical filter inserted.

FIG. 40 is a schematic perspective view schematically showing an opticalfilter 302 installed in the waveguide-embedded optical circuit 300, andFIG. 41 is a cross-sectional view taken along line E-E of FIG. 40. Asshown in FIG. 40 and FIG. 41, the beam propagating through the waveguideconstituting one side of the spot-size transformer 200-1 and 200-2 isfiltered according to the characteristic of the optical filter 302inserted into the groove 301 and transmits to the waveguide constitutingthe other side of the spot-size transformer 200-1 and 200-2. Thereby, itis possible to extract a light of desired wavelength.

In the waveguide-embedded optical circuit 300 of this embodiment, sincethe portion separated by the groove 301 serves as a second opticalwaveguide with a core of larger diameter than the first opticalwaveguide, the beam spot propagating through the groove 301 is expandedto larger than the beam spot which propagates through the firstwaveguide. As a result, it is possible to significantly reduce thediffraction loss arising in a groove.

As explained above, since the waveguide-embedded optical circuit 300 isconfigured using a pair of the spot-size transformers 200 arranged sothat the end faces 200 b face each other across a groove 301 and theoptical filter 302 is inserted into the groove 301, the optical filtercircuit with low loss can be realized.

The optical functional element inserted into such a groove 301 is ofcourse not limited to an optical filter, and an optical isolator circuitor an optical circulator circuit can be constituted by inserting anoptical isolator element including Faraday rotator or the like. Forexample, as shown in FIG. 42, a low-loss optical isolator element can beconstituted by inserting into the groove an optical isolator element 310equipped with a Faraday rotator 311 that rotates polarization 45 degreesand polarizers 312, 313 of different transmittance polarizationdirections provided on two surfaces of the Faraday rotator 311 to faceeach other and applying a magnetic field along the direction of theoptical axis.

Further, as shown in FIG. 43, in an arrayed waveguide-embedded opticalcircuit 400 which comprises several pairs of the first embedded opticalwaveguide and the second embedded optical waveguide facing each otheracross a groove 410, since the optical transformer 200 is provided oneach of the first embedded optical waveguide and the second embeddedoptical waveguide, a low loss arrayed waveguide-embedded optical circuitcan be realized.

Therefore, a low loss optical filter array can be realized by insertingan optical filter into the groove 410 shown in FIG. 43. In this case,the same filtering characteristic can be imparted to every channel byinserting a large optical filter into the groove 410, or insertingseveral optical filters corresponding to one or more channels.

In addition, an optical isolator element array can be constituted byinserting an optical isolator element into the groove 410 shown in FIG.43. In this case, by inserting an optical isolator element in aprescribed part of the groove 410 and inserting an optical filter inremaining part of the groove 410, it can be made serve as an opticalisolator circuit with respect to a certain channel or channels and toserve as an optical filter circuit with respect to the remaining channelor channels.

Further, as shown in FIG. 44, in the waveguide-embedded opticalwaveguide 500, which comprises a embedded optical waveguide 501-508, agroove 510 which separates the embedded optical waveguide 505 and theembedded optical waveguide 507 and separates the embedded opticalwaveguide 506 and the embedded optical waveguide 508 and opticalcombining/dividing members 511 and 512 and constitutes aninterferometer, if a non-reciprocal element (not shown) consisting of aFaraday rotator which rotates polarization 45 is inserted into thegroove 510 and a birefringent element provided on either side of theFaraday rotator, an optical circulator can be constituted.

The present invention has thus been shown and described with referenceto specific embodiments. However, it should be noted that the presentinvention is in no way limited to the details of the describedarrangements but changes and modifications may be made without departingfrom the scope of the appended claims.

For example, in the above-described, the optical resin layer 105 isformed in the part corresponding to the first optical waveguide.However, since the optical resin layer 105 serves as the second opticalwaveguide (second cladding), it can be omitted in the part correspondingto the first optical waveguide. And it is not necessary to use theoptical resin layer 105 as the second cladding insofar as the refractiveindex of the optical resin layer 105 is different from the refractiveindex of the lower cladding layer 102 and the upper cladding layer 103and other materials may be used.

In the above-described embodiments, the optical filter 110 is insertedinto the groove 106 formed in the waveguide-embedded optical circuit100. However, optical filters which can be inserted into the groove 106are not limited to the optical filter 110 and other kinds of opticalfilters such as a Faraday rotator or the like may be used.

In the above-described embodiments, the substrate 101 is formed usingsilica glass. However, the substrate 101 is not limited to silica glassinsofar as a beam can be effectively confined in the lower claddinglayer 102 and the substrate 101 may be formed by of silicon that hassilica glass layer on the surface.

In the above-described embodiments, the groove 106 is formed by dicingwith a dicing machine. However, the groove 106 may be formed by forminga metal mask layer over the whole of the upper cladding layer 103 andthe optical resin layer 105, applying spin-coating a photoresist on themetal mask layer, forming an etching mask for leaving parts other thanthat corresponding to the groove 106 and removing unnecessary portionsof the optical resin layer 105, the lower cladding layer 102, the uppercladding layer 103 and the part of the substrate 101 using the etchingmask.

In the above-described embodiments, silica glass or a material whichincludes silica as the principal ingredient is used as the material ofcomponents of the spot-size transformer 200 (e.g. upper cladding layer202). However, other materials may be used insofar as the componentscorresponding to the lower cladding layer 202 and upper cladding layer203 are formed using a material which changes its refractive index uponirradiation by a high-energy beam such as ultraviolet rays. Variousphotopolymers are known to undergo a change of that shows refractiveindex change upon exposure to a high-energy beam such as ultravioletrays are known. One such photopolymer is silicone doped branchedpolysilane. When such organic materials are used, it is preferable forit to be spin-coated.

In the above-described embodiments, non-doped silica glass (SiO₂) isused as the material of the substrate 201. However, silicon or the likemay be used on instead of the SiO₂ substrate and non-doped silica glass(SiO₂) may be formed on the surface of the substrate and used as part ofthe second cladding.

EXAMPLE 1

The part of the spot-size transformer seen from the groove 106 of thewaveguide-embedded optical circuit 100 of the above-mentionedembodiment, that is, the spot-size transformer, was fabricated of onlythe substrate 101, the lower cladding layer 102-1-102-3, the uppercladding layer 103-1- 103-3, the core region 104-1 and the optical resinlayer 105-1. A silica glass containing germanium was used as thematerial of the core region 104-1 (first core) and BPSG (silica glassdoped with boron and phosphorus; n=1.4558) was used as the material ofthe lower cladding layer 102-2 and the upper cladding layer 103-2 (firstcladding=second core) and the same may be said of the lower claddinglayer 102-1, 102-3, and the upper cladding layer 103-1, 103-3 and anoptical adhesive (n=1.4473) was used as the optical resin layer 105-1(second cladding).

Furthermore, as the size of the core region 104-1 (first core), thelength of the region corresponding to the first optical waveguide wasset to 200 μm and the width and the length of the region was set to 7 μmand the length x2 of the taper corresponding to the transition waveguidewas set to 1000 μm and the width x1 of the end of the taper was set to0.4 μm.

Furthermore, in the layered members consisting of the lower claddinglayer 102-2 and the upper cladding layer 103-2 (first core=second core),the length, the height, and the width of the layered members were 2400μm, 35 μm, 34 μm, respectively. The 200 μm section in which the heightand the width of the core region 104-1 of the first core is set constant(the part corresponding to the first optical waveguide) served as thefirst cladding layer and the 1200 μm section where the core region 104-1is not present served as the second core (the part corresponding to thesecond optical waveguide). And the 1000 μm section in which the coreregion 104-1 was tapered (the part corresponding to the transitionwaveguide) gradually changed in function from that of the first claddinglayer to that of the second core.

A beam having the optical field mode distribution shown in FIG. 45(spot-size=about 10 μm) and the beam was input to the first opticalwaveguide of the spot-size transformer of such structure and opticalfield mode distribution of the beam output from the second opticalwaveguide was measured. The optical field mode distribution of the beamoutput from the second optical waveguide is shown in FIG. 46. As shownin FIG. 46, it was found that the spot-size of the beam output from thesecond optical waveguide was about 28 μm, meaning that it had beenenlarged 2.8 times.

EXAMPLE 2

A waveguide-embedded optical circuit was fabricated in the same manneras in the Example 1. The two spot-size transformers included in thewaveguide-embedded optical circuit in accordance with the Example 2 wereof the same material and in the same size as the one in accordance withthe Example 1. The width of the slit separating the two spot-sizetransformers was set to 400 μm.

On the other hand, as a comparative example, a waveguide-embeddedoptical circuit was fabricated in which the core region 104-1 was nottapered and the height and width of the core region was fixed at 7 μm.Since the spot-size was not transformed in the waveguide-embeddedoptical circuit according to the comparative example and all regionscorresponded to the first optical waveguide, the optical resin layer 105was not provided. That is, the waveguide-embedded optical circuitaccording to the comparative example corresponded to the conventionalwaveguide-embedded optical circuit shown FIG. 3. In thewaveguide-embedded optical circuit according to the comparative example,the width of the slit separating the two spot-size transformers was setto 400 μm.

The excessive loss of a beam propagated through the groove was measuredfor the waveguide-embedded optical circuits according to the Example 2and the comparative example. The excessive loss was 0.33 dB in thewaveguide-embedded optical circuit according to the Example 2 and 8.1 dBin the waveguide-embedded optical circuit according to the comparativeexample, thus confirming that the diffraction loss by the spot-sizetransformer was reduced significantly in the optical circuit accordingto the invention.

EXAMPLE 3

A spot-size transformer having the structure of the spot-sizetransformer 200 shown from FIG. 23 to FIG. 28 was produced.

First, the substrate 201 was formed of silica (n=1.4460) to a thicknessof about 1 mm (see FIG. 29). Next, the lower cladding layer 202 wasformed of GBSG (n=1.4660) to a thickness of 14 μm and the core layer204′ was formed of GBPSG (n=1.4517) to a thickness of 7 μm. These layerswere formed on the surface of the substrate 201 in the order mentionedby the CVD process (see FIG. 30), and the core region 204 was formed bypattering the core layer 204′ (see FIG. 31). As the size of the coreregion 204 (first core), the length of the region corresponding to thefirst optical waveguide was set to 200 μm and the width of the regionwas set to 7 μm and the length x₂ of the taper corresponding to thetransition waveguide was set to 1000 μm and the width x₁ of the end ofthe taper was set to 0.4 μm.

Next, the upper cladding layer 203 consisting of GBSG (n=1.4460) of athickness of 17 μm was formed on the surface of the lower cladding layer202 and the core layer 204 by the CVD process (see to FIG. 32). Then, byannealing at 1100° C. for 24 hours, the flow of the upper cladding layer203 was flowed to flatten. By the annealing process, the thickness ofthe upper cladding layer 203 decreased to 14 μm. Then, the topmostcladding layer 206 consisting of GBSG (n=1.4460) of a thickness of 17 μmwas formed on the surface of the upper cladding layer 203 by the CVDprocess (see FIG. 33).

Next, with the sputtering process, the metal mask layer 207′ was formedon the topmost cladding layer 206 to a thickness of 1 μm using WS (seeFIG. 34) and a metal mask 207 was pattered the metal mask layer 207′(see FIG. 35). The width of the aperture of the metal mask 207 was setto 34 μm. Next, a refractive index changing region was formed byirradiation of a KrF excimer laser beam (pulse power=10 mJ/cm²) throughthe aperture of the metal mask 207 onto the lower cladding layer 202 andupper cladding layer 203 through the topmost cladding layer 206 forabout 10 hours, with the wavelength of the laser set to 248 nm and thepulse frequency set to 20 Hz (see FIG. 36). As a result, the refractiveindex of the refractive index changing region was adjusted to 1.4485. Inaddition, the refractive index of the core region 204 also was increasedto about 1.4542.

The metal mask 207 was removed from on the topmost cladding layer 206 tocomplete, then the spot-size transformer in accordance with Example 3was completed.

The length of the refractive index changing region 205 was 2300 μm, andthe width and the height thereof were 34 μm and 35 μm, respectively. The200 μm section in which the height and the width of the core region 204of the first core was set constant (the part corresponding to the firstoptical waveguide) served as the first cladding and the 1100 μm sectionwhere the core region 204 was not present (the part corresponding to thesecond optical waveguide) served as the second cladding. And the 1000 μmsection in which the core region 204 was tapered (the part correspondingto the transition waveguide) gradually changed in function from that ofthe first cladding to that of the second core.

A beam having the optical field mode distribution shown in FIG. 47(spot-size=about 10 μm) and a wavelength of 1550 μm was input to thefirst optical waveguide of the spot-size transformer of this structure,and optical field mode distribution of the beam output from the secondoptical waveguide was measured. The optical field mode distribution ofthe beam output from the second optical waveguide is shown in FIG. 48.As shown in FIG. 48, the spot-size of the beam output from the secondoptical waveguide was found to be about 28 μm, meaning that it had beenenlarged 2.8 times.

EXAMPLE 4

A waveguide-embedded optical circuit having the structure of thewaveguide-embedded optical circuit 300 shown in FIG. 37 to FIG. 39 wasproduced. The two spot-size transformers included in thewaveguide-embedded optical circuit in accordance with Example 3 were ofthe same material and same size as the one in accordance with theExample 3, and differed therefrom only in that the irradiating energy ofthe KrF excimer laser was set to various values. The width of the grooveseparating the two spot-size transformers was set to 400 μm, and anoptical resin having a refractive index of 1.447 was filled in thegroove. As for the irradiating energy of the KrF excimer laser, thepulse power was set to 40 mJ/cm² and the frequency to 20 Hz. Theirradiation time was changed for each sample.

The relation between the total irradiation energy of the KrF excimerlaser and insertion loss was evaluated by transmitting the light withthe wavelength of 1550 μm from one spot-size transformer of the firstoptical waveguide to the other spot-size transformer of the firstoptical waveguide for each sample.

FIG. 49 is a graph showing the relation between the total irradiationenergy of the KrF excimer laser and insertion loss. As shown in FIG. 49,when the total energy of the KrF excimer laser was 6 mJ/cm²-12 mJ/cm²,the insertion loss became small, and in particular, when it was 8mJ/cm², the insertion loss became smallest (0.51 dB). It was found thatwhen the materials mentioned were used to constitute the components ofthe spot-size transformer, it was preferable to set the irradiationenergy of the KrF excimer laser at 6 mJ/cm²-12 mJ/cm², and morepreferable to set it at about 8 mJ/cm².

On the other hand, as a comparative example a waveguide-embedded opticalcircuit was fabricated in which the core region 204 was not tapered andthe height and the width of the core region 204 was fixed at 7 μm. Sincethe spot-size was not transformed in the waveguide-embedded opticalcircuit according to the comparative example and all regionscorresponded to the first optical waveguide, formation of the refractiveindex changing region by irradiating ultraviolet ray was not formed. Inthe waveguide-embedded optical circuit according to the comparativeexample, the width of the groove separating the two spot-sizetransformers was set to 400 μm, and an optical resin which had arefractive index of 1.447 was filled in the groove. When a light with awavelength of 1550 μm was transmitted through the waveguide-embeddedoptical circuit of the comparative example, the insertion loss wasbecame very large (8.1 dB).

1. A spot-size transformer comprising: a first optical waveguide havinga first core and a first cladding having an upper and a lower claddingof a same material covering substantially a whole surface of the firstcore, the first core having a surface region and an axis and the firstcladding having an axis; a second optical waveguide having a secondcladding and a second core, the second core being a continuation of theupper and the lower cladding of the first cladding; a transition regionbetween the first and second optical waveguides, the transition regionhaving an extension of the first core whose width becomes graduallynarrower as it extends toward the second optical waveguide.
 2. Thespot-size transformer in accordance with claim 1, wherein each of thefirst optical waveguide and the second optical waveguide ischannel-type.
 3. The spot-size transformer in accordance with claim 2,wherein a center of the first core and a center of the second core arepositioned substantially on a common axis.
 4. The spot-size transformerin accordance with claim 3, wherein the first cladding has at least alower cladding positioned under the first core and an upper claddingpositioned above the first core.
 5. The spot-size transformer inaccordance with claim 4, wherein a bottom of the first core is incontact with the lower cladding and a top surface and both sides of thefirst core are in contact with the upper cladding.
 6. The spot-sizetransformer in accordance with claim 5, wherein an end face of thesecond core is substantially rectangular.
 7. The spot-size transformerin accordance with claim 1, wherein a section with the first claddingand a part constituting the second core that is an extension thereofsubstantially perpendicular to the axis of the first core arerectangular.
 8. The spot-size transformer in accordance with claim 2,wherein a section with the first cladding and a part constituting thesecond core that is an extension thereof substantially perpendicular tothe axis of the first core are rectangular.
 9. The spot-size transformerin accordance with claim 3, wherein a section with the first claddingand a part constituting the second core that is an extension thereofsubstantially perpendicular to the axis of the first core arerectangular.
 10. The spot-size transformer in accordance with claim 4,wherein a section with the first cladding and a part constituting thesecond core that is an extension thereof substantially perpendicular tothe axis of the first core are rectangular.
 11. The spot-sizetransformer in accordance with claim 5, wherein a section with the firstcladding and a part constituting the second core that is an extensionthereof substantially perpendicular to the axis of the first core arerectangular.
 12. The spot-size transformer in accordance with claim 6,wherein a section with the first cladding and a part constituting thesecond core that is an extension thereof substantially perpendicular tothe axis of the first core are rectangular.
 13. The spot-sizetransformer in accordance with claim 12, wherein the first core has ashape obtained by omitting the extension of the first core whose widthbecomes gradually narrower.
 14. The spot-size transformer in accordancewith claim 13, wherein the second cladding is formed of a laddersilicone.
 15. The spot-size transformer in accordance with claim 13,wherein the second cladding is formed of a silica glass.
 16. Thespot-size transformer in accordance with claim 15, wherein the secondcladding is formed using a thin film process selected from a groupconsisting of a CVD process, a sputtering process, a vacuum depositionprocess, a FHD process and a sol-gel process.
 17. A spot-sizetransformer, comprising: a first optical waveguide having a first coreand a first cladding having an upper and a lower cladding of a samematerial covering the first core; a second optical waveguide having asecond core and a second cladding covering the second core, wherein thesecond core is a continuation of the upper and the lower cladding of thefirst cladding; a transition waveguide which is positioned between thefirst and the second optical waveguides; wherein a light propagated intothe first optical waveguide has a first optical field distribution;wherein a light propagated into the second optical waveguide has asecond optical field distribution; wherein the transition waveguidechanges from the first optical field to the second optical fieldgradually or changes from the second optical field to the first opticalfield gradually; and wherein the second core covers the first core atleast in a part corresponding to the transition waveguide and includes aregion where a refractive index is changed by irradiating energy beam.18. The spot-size transformer in accordance with claim 17, wherein awidth of the part of the first core that corresponds to the transitionwaveguide becomes gradually narrower as it goes toward the secondoptical waveguide.
 19. The spot-size transformer in accordance withclaim 18, wherein a part of at least the first cladding is provided asan extension of the second core.
 20. The spot-size transformer inaccordance with claim 19, wherein the second cladding has a first partwhich comprises substantially non-doped silica glass and a second partwhich comprises silica glass containing at least germanium (Ge).
 21. Thespot-size transformer in accordance with claim 20, wherein the secondpart further contains a first element which reduces refractive index.22. The spot-size transformer in accordance with claim 21, whereinrefractive indexes of the first part and the second part aresubstantially equal.
 23. The spot-size transformer in accordance withclaim 22, wherein the first core comprises a material in which at leastgermanium (Ge), a first element and a second element which raise itsrefractive index are contained in silica glass.
 24. The spot-sizetransformer in accordance with claim 23, wherein the first element isboron (B) and the second element is phosphorus (P).
 25. The spot-sizetransformer in accordance with claim 24, wherein the first opticalwaveguide and the second optical waveguide are channel-type and a centerof the first core and a center of the second core are locatedapproximately on a same axis.